Fiber Bragg grating interrogation and sensing system and methods comprising a filter centered at a first wavelength

ABSTRACT

Fiber Bragg grating interrogation and sensing used for strain and temperature measurements. A simple, broadband light source is used to interrogate one or more fiber Bragg grating (FBG). Specifically, a packaged LED is coupled to fiber, the light therefrom is reflected off a uniform FBG. The reflected light is subsequently analyzed using a filter and a plurality of Si photodetectors. In particular, the filter is a chirped FBG or an optically coated filter, in accordance with some embodiments. Measurement analysis is performed by ratio of intensities at the plurality of detectors, at least in part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/136,223 entitled, “FIBER BRAGG GRATING INTERROGATION AND SENSINGSYSTEM AND METHODS COMPRISING A FIRST PHOTODETECTOR FOR MEASURINGFILTERED LIGHT AND A SECOND PHOTODETECTOR FOR MEASURING UNFILTEREDLIGHT” now U.S. Pat. No. 10,663,325 which is related and claims priorityto U.S. Provisional Application No. 62/560,593 entitled, “FIBER BRAGGGRATING INTERROGATION AND SENSING SYSTEM AND METHODS” filed on Sep. 19,2017, all of which are hereby incorporated by reference in its entirety.This application is also related to U.S. Pat. Nos. 8,538,215 and9,348,088 entitled, “Optical Package and Related Methods” and “Systemsand methods for passive alignment of opto-electronic components,”respectively, which are hereby incorporated by reference in theirentirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to fiber Bragg grating sensing andinterrogation used in strain measurement. More specifically, thisdisclosure describes apparatus and techniques relating to using acommonly available interrogator source and methods of measurementthereof.

BACKGROUND

Fiber Bragg grating (FBG) sensors have been playing a vital role in manyindustrial applications owing to their high sensitivity, fast response,immunity to electromagnetic interference, high reliability, distributedand multiplexing capability, and multiparameter sensing. The past decadehas seen a tremendous growth in the number of FBG-based sensor systems.Nowadays, temperature and strain measurement encompass a wide variety ofneeds and applications in scientific fields, aerospace, metallurgicaland civil engineering, solar panels, nuclear power, shipping, petroleum,and thermal power industries.

A fiber Bragg grating (FBG) is a type of distributed Bragg reflectorconstructed in a short segment of optical fiber that reflects particularwavelengths of light and transmits all others. This is achieved byadding a periodic variation to the refractive index of the fiber core,which generates a wavelength specific dielectric mirror. A fiber Bragggrating can therefore be used as an inline optical filter to blockcertain wavelengths, or as a wavelength-specific reflector.

Fiber Bragg Gratings are made by laterally exposing the core of asingle-mode fiber to a periodic pattern of intense ultraviolet light.The exposure produces a permanent increase in the refractive index ofthe fiber's core, creating a fixed index modulation according to theexposure pattern. This fixed index modulation is called a grating. Ateach periodic refraction change a small amount of light is reflected.All the reflected light signals combine coherently to one largereflection at a particular wavelength when the grating period isapproximately half the input light's wavelength. This is referred to asthe Bragg condition, and the wavelength at which this reflection occursis called the Bragg wavelength. Light signals at wavelengths other thanthe Bragg wavelength, which are not phase matched, are essentiallytransparent.

Sensing technologies based on optical fiber have several inherentadvantages that make them attractive for a wide range of industrialsensing applications. They are typically small in size, passive, immuneto electromagnetic interference, resistant to harsh environments andhave a capability to perform distributed sensing. Because of theirtelecommunication origins, fiber optic-based sensors can be easilyintegrated into large scale optical networks and communications systems.

Although developed initially for the telecommunications industry in thelate 1990's, fiber Bragg gratings (FBGs) are increasingly being used insensing applications and are enjoying widespread acceptance and use. TheFBG is an optical filtering device that reflects light of a specificwavelength and is present within the core of an optical fiber waveguide.The wavelength of light that is reflected depends on the spacing of aperiodic variation or modulation of the refractive index that is presentwithin the fiber core. This grating structure acts as a band-rejectionoptical filter passing all wavelengths of light that are not inresonance with it and reflecting wavelengths that satisfy the Braggcondition of the core index modulation. The Nobel Laureate Sir WilliamLawrence Bragg established the Bragg law in 1915, describing with asimple mathematical formula how X-Rays were diffracted from crystals.The Bragg condition, when applied to fiber Bragg gratings, states thatthe reflected wavelength of light from the grating is:λ_(B)=2·n _(eff)·Λ_(G)

where, n_(eff) is the effective refractive index seen by the lightpropagating down the fiber and Λ_(G) is the period of the indexmodulation that makes up the grating.

Fiber Bragg gratings (FBGs) are widely used as sensing elements for themeasurement of physical parameters such as strain, pressure andtemperature. The variation of these parameters induces changes of thecentral Bragg wavelength. The precise measurement of this wavelengthchange is crucial for achieving good sensor performance.

Several interrogation techniques based on bulk filters, fiber edgefilters, edge optical spectra, edge fiber grating spectra, edge detectorspectral responses, tunable fiber filters, tunable acousto-opticfilters, tunable single mode laser diodes, receiving FBGs,interferometric detection, fiber lasers and Fourier techniques have beenproposed.

An FBG illuminated by a broadband light source reflects a particularnarrow band wavelength called Bragg wavelength and transmits all others.The Bragg wavelength is mainly dependent on applied temperature andstrain. In general, the Bragg wavelength shift of FBG is monitored byusing an optical spectrum analyzer (OSA). However, OSA has its ownlimitations in response time, resolution, weight, size, and cost. Toovercome these issues, different interrogation techniques have beendeveloped.

A more recent advance is the use of a simple technique based onconverting the wavelength information into its equivalent intensitymodulated signal which can be measured by using a photodiode with simpleelectronics. However, the inventor of the present disclosure hasrecognized the need for a more robust, low cost FBG sensor whichexploits this technique.

This overview is intended to provide an overview of subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. Furtherlimitations and disadvantages of conventional and traditional approacheswill become apparent to one of skill in the art, through comparison ofsuch systems with some aspects of the present invention as set forth inthe remainder of the present application with reference to the drawings.

SUMMARY OF THE DISCLOSURE

Fiber Bragg grating interrogation and sensing used for strain andtemperature measurements. A simple, broadband light source is used tointerrogate one or more fiber Bragg grating (FBG). Specifically, apackaged LED is coupled to fiber, the light therefrom is reflected off auniform FBG. The reflected light is subsequently analyzed using a filterand a plurality of Si photodetectors. In particular, the filter is achirped FBG in accordance with some embodiments.

In other embodiments, the filter is an optical coated filter.Measurement analysis is performed by ratio of intensities at theplurality of detectors, at least in part. According to one aspect, thepresent disclosure is a system for measuring strain or temperaturecomprising a broadband light source, one or more splitter/couplers, afiber Bragg grating, an optical filter and 2 or more optical detectors.

According to one aspect of the disclosure, a method for interrogating afiber Bragg grating is disclosed. The method comprises powering a lightsource with a current, illuminating a fiber Bragg grating with a lightpulse, and reflecting a portion of the light pulse centered a firstwavelength λ₁.

According to another aspect of the disclosure, the method furthercomprises separating the portion of the light pulse centered at a firstwavelength λ₁ into a first and second intensity.

According to another aspect of the disclosure, the method furthercomprises filtering the first intensity with a filter centered at asecond wavelength λ₂.

According to another aspect of the disclosure, the method furthercomprises measuring the filtered first intensity and measuring thesecond intensity.

According to another aspect of the disclosure, the method furthercomprises calculating a change in the fiber Bragg grating using themeasurement of the first and second intensity.

According to another aspect of the disclosure, the method furtherqualifies that the light source comprises a first light emitting diodehave a spectral intensity centered about a third wavelength, λ₃.

According to another aspect of the disclosure, the method furtherqualifies that the light source further comprises a second lightemitting diode have a spectral intensity centered about a fourthwavelength, λ₄.

According to another aspect of the disclosure, the method furtherqualifies that the light source further comprises a second lightemitting diode have a spectral intensity centered about a fourthwavelength, λ₄.

According to another aspect of the disclosure, the method furtherqualifies that the filtering the first intensity with a filter centeredat a second wavelength λ₂ is performed with an optically coated filter.

According to another aspect of the disclosure, the method furtherqualifies that the filtering the first intensity with a filter centeredat a second wavelength λ₂ is performed with a chirped fiber Bragggrating.

According to another aspect of the disclosure, the method furthercomprising calculating a ratio of the first and second intensity.

According to another aspect of the disclosure, the method furtherqualifies that an analog front-end performs the calculation.

According to another aspect of the disclosure, the method furtherqualifies that the ratio is calculated by the following:

$R = \frac{I_{D\; 1} - I_{D2}}{I_{D1} + I_{D2}}$

where, I is the current received from two detectors D₁ and D₂.

According to another aspect of the disclosure, the method furtherqualifies that the measuring the filtered first intensity and measuringthe second intensity is performed using a first photodetector and secondphotodetector, respectively.

According to another aspect of the disclosure, the system forinterrogating a fiber Bragg grating is disclosed comprising a lightsource, a fiber Bragg grating centered at a first wavelength λ₁ which isilluminated by the light source, and a filter centered at a secondwavelength λ₂, the filter filters the light reflected from the fiberBragg grating.

According to another aspect of the disclosure, the system furthercomprises a first photodetector, wherein the first photodetectormeasures the filtered light.

According to another aspect of the disclosure, the system furthercomprises a second photodetector, wherein the second photodetectormeasures unfiltered light.

According to another aspect of the disclosure, the system furthercomprising an analog front-end configure to perform calculations on themeasurements from the first and second photodetector.

According to another aspect of the disclosure, the system furtherqualifies that the calculations include estimating a ratio from thefirst and second photodetector.

According to another aspect of the disclosure, the system furtherqualifies that the wherein the ratio is calculated by the following:

$R = \frac{I_{D1} - I_{D2}}{I_{D1} + I_{D2}}$

where I is the current received from two detectors D₁ and D₂.

According to another aspect of the disclosure, the system furtherqualifies that the calculations include calculating a change in thefiber Bragg grating using the measurement of the first and secondphotodetector and predetermined properties of the filter.

According to another aspect of the disclosure, the system furtherqualifies that the first and second photo detectors are photodiodes.

According to another aspect of the disclosure, the system furtherqualifies that the light source in a light emitting diode.

According to another aspect of the disclosure, the system furtherqualifies that the filter is a chirped fiber Bragg grating.

According to another aspect of the disclosure, the system furtherqualifies that the filter is an optical interference filter.

According to another aspect of the disclosure, the system furtherqualifies that the filter is an optical dichroic filter.

According to one or more aspects of the present disclosure, an apparatusfor interrogating a fiber Bragg grating is disclosed comprising a meansfor powering a light source with a current, a means for illuminating afiber Bragg grating with a light pulse, and a means for reflecting aportion of the light pulse centered a first wavelength λ₁.

According to another aspect of the disclosure, the apparatus furthercomprises a means for separating the portion of the light pulse centeredat a first wavelength λ₁ into a first and second intensity.

According to another aspect of the disclosure, the apparatus furthercomprises a means for filtering the first intensity with a filtercentered at a second wavelength λ₂.

According to another aspect of the disclosure, the apparatus furthercomprises a means for measuring the filtered first intensity and a meansfor measuring the second intensity.

According to another aspect of the disclosure, the apparatus furthercomprises a means for calculating a change in the fiber Bragg gratingusing the measurement of the first and second intensity.

According to another aspect of the apparatus in the present disclosure,the broadband light source is a light emitting diode (LED).

According to another aspect of the apparatus in the present disclosure,the system is for interrogating a fiber Bragg grating sensor.

According to another aspect of the apparatus in the present disclosure,the splitter/coupler are optical circulators.

According to another aspect of the apparatus in the present disclosure,the FBG is uniform.

According to another aspect of the apparatus in the present disclosure,the optical filter is a chirped FBG.

According to another aspect of the apparatus in the present disclosure,the optical filter is an add/drop filter.

According to another aspect of the apparatus in the present disclosure,the filter is a dichroic mirror.

According to another aspect of the apparatus in the present disclosure,the plurality of optical detectors are Silicon photodetectors.

According to yet another aspect of the present disclosure, a method fordetermining FBG sensing measurement comprises edge detection.

According to yet another aspect of the present disclosure, a method fordetermining FBG sensing measurement comprises ratio of intensities.

The drawings show exemplary FPGs circuits, systems and configurations.Variations of these systems, for example, changing the positions of,adding, or removing certain elements from the circuits are not beyondthe scope of the present invention. The illustrated FPG devices andconfigurations are intended to be complementary to the support found inthe detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of the presentinvention, reference is made to the following detailed description ofpreferred embodiments and in connection with the accompanying drawings,in which:

FIG. 1 shows an exemplary fiber Bragg grating fiber includingreflectance and transmission response as a function of wavelength, inaccordance with some embodiments of the disclosure provided herein;

FIG. 2 shows an exemplary chirped fiber Bragg grating fiberdemonstrating effect on multiple wavelengths, in accordance with someembodiments of the disclosure provided herein;

FIG. 3 depicts an exemplary fiber Bragg grating system comprisingsensors and interrogator, in accordance with some embodiments of thedisclosure provided herein;

FIG. 4 illustrates exemplary spectra from the source and resultantreflections at two different temperatures, in accordance with someembodiments of the disclosure provided herein;

FIG. 5 illustrates exemplary spectra from resultant reflections at twodifferent temperatures juxtaposed to broad band response of an exemplarychirped fiber Bragg grating fiber, in accordance with some embodimentsof the disclosure provided herein;

FIG. 6 demonstrates the decomposition of incident exemplary spectra onan exemplary chirped fiber Bragg grating juxtaposed to broad bandresponse thereof, the decomposition comprising transmission andreflection, in accordance with some embodiments of the disclosureprovided herein;

FIG. 7 depicts an exemplary fiber Bragg grating system comprisingsensors, interrogator, and optical circulators, in accordance analternate embodiment of the disclosure provided herein;

FIG. 8 depicts an exemplary fiber Bragg grating system comprisingsensors and interrogator, in accordance with some embodiments of thedisclosure provided herein;

FIG. 9 depicts an exemplary fiber Bragg grating system comprisingsensors, interrogators, an interrogated FBG and reference FBG, inaccordance with some embodiments of the disclosure provided herein;

FIG. 10 depicts an exemplary fiber Bragg grating system comprisingsensors, interrogators and a plurality of FBGs, in accordance with someembodiments of the disclosure provided herein;

FIG. 11 illustrates packaging for an exemplary fiber Bragg gratingsystem comprising sensors, interrogators and AFE, in accordance withsome embodiments of the disclosure provided herein;

FIG. 12 illustrates packaging for an exemplary fiber Bragg grating fiberinterface, in accordance with some embodiments of the disclosureprovided herein;

FIG. 13 illustrates packaging detection for an exemplary fiber Bragggrating system comprising sensors and optical filters, in accordancewith some embodiments of the disclosure provided herein; and,

FIG. 14 shows an exemplary fiber Bragg grating sensing filter, inaccordance an alternate embodiment of the disclosure provided herein.

DETAILED DESCRIPTION

The present disclosure relates to fiber Bragg grating sensing andinterrogation used in strain measurement. More specifically, thisdisclosure describes apparatus and techniques relating to using acommonly available interrogator source and methods of measurementthereof.

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrative examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure are set forthin the proceeding in view of the drawings where applicable.

Because of their small size, passive nature, immunity to electromagneticinterference, and capability to directly measure physical parameterssuch as temperature and strain, fiber Bragg grating sensors havedeveloped beyond a laboratory curiosity and are becoming a mainstreamsensing technology.

Fiber Bragg Gratings and other optical transducers convert physicalchanges of displacement or bending or stretching to change in thereflectance or the transmission spectrum. Many types of such transducershave been invented and they range from Fabry-Perot interferometers tochirped linear gratings to fiber Bragg gratings (FBG). FBG's areespecially popular because they are robust, easy to install, and can bedesigned to provide reflectance or transmission depending on theapplication. The disadvantage of the FBG's—and of many other opticaltransducers—is that they provide a relatively small change in thespectrum (either in reflection or transmission mode) relative to thechange in strain or temperature or any other physical parameter thatchanges the Bragg structure. This generally requires that a highwavelength resolution to measure changes. Many methods to measure thesechanges are proposed in the present disclosure.

FBG is used as an illustrative example of providing highly sensitivetransducer to convert changes in the environment to changes in thewavelength spectrum. The technique presented here can be used with manyother optical devices that perform the same function such as grating ora Fabry-Pero interferometer.

FIG. 1 shows an exemplary fiber Bragg grating fiber 100 includingreflectance and transmission response, 180, 190, respectively as afunction of wavelength, in accordance with some embodiments of thedisclosure provided herein. A fiber Bragg grating (FBG) is amicrostructure typically a few millimeters in length that can be photoinscribed in the core 130 of a single mode fiber. This is done bytransversely illuminating the fiber with a UV laser beam and using aphase mask to generate an interference pattern in its core.

This will induce a permanent change in the physical characteristics ofthe silica matrix. This change consists in a spatial periodic modulationof the core 130 index of refraction that creates a resonant structure.Protected with a primary coating/buffer 110, the diameter of the totalfiber is 250 micrometers. Without this coating, the fiber has a diameterof 125 micrometers measured around the cladding 120. The light thentravels within the core 130, which has a diameter of approximately 8micrometers.

A fiber Bragg grating (FBG) is a type of distributed Bragg reflectorconstructed in a short segment of optical fiber that reflects particularwavelengths of light and transmits all others. This is achieved bycreating a periodic variation 140 in the refractive index 160 of thefiber core 130, which generates a wavelength-specific dielectric mirror.A fiber Bragg grating 100 can therefore be used as an inline opticalfilter to block certain wavelengths, or as a wavelength-specificreflector.

As a resonant structure, the FBG 100 will act as a wavelength selectivemirror; it is a narrow band filter. This means that if light from abroadband source 170 is injected in the optical fiber 100, only lightwithin a very narrow spectral width AB centered at the Bragg wavelengthwill be back-reflected by the grating, as illustrated by the reflectanceresponse 180. The remaining light will continue through the opticalfiber to the next Bragg grating (not shown) without experiencing anyloss. The Power transmitted is depicted in 190 through the optical fiber100.

The fundamental principle behind the operation of an FBG is Fresnelreflection, where light traveling between media of different refractiveindices may both reflect and refract at the interface. The refractiveindex will typically alternate over a defined length. The reflectedwavelength, λ_(B), called the Bragg wavelength, is defined by therelationship, λ_(B)=2·n_(eff)·Λ_(G), where, n_(eff) is the effectiverefractive index seen by the light propagating down the fiber and λ_(G)is the period of the index modulation that makes up the grating.

The effective refractive index quantifies the velocity of propagatinglight as compared to its velocity in vacuum. n_(eff) depends not only onthe wavelength but also (for multimode waveguides) on the mode in whichthe light propagates. For this reason, it is also called modal index.

Turning to FIG. 1, Λ represents the periodicity in a uniformed fiberBragg grating 100. In general, n₃>n₂>n₁>n₀. This engenders a conditionwhereby FPG acts a wavelength dependent mirror while not allowing energyto escape through the cladding (i.e., total internal reflection). Theincident power, PI, decomposes in to two constituent parts, P_(R) andP_(T), where P_(R) and P_(T) is the power reflected and the powertransmitted, respectively. Light centered around λ_(B) is reflected backtowards in ingress of the FBG. Conversely, all other light not centeredaround λ_(B) passes through the FBG and egresses.

The Bragg wavelength λ_(B) is defined by the period 140 of themicrostructure and the index of refraction 160 of the core 130. Atypical FBG is a symmetric structure, so it will always reflect light atthe Bragg wavelength no matter which side the light is coming from.

Fiber Bragg gratings are created by inscribing or writing systematic(periodic or aperiodic) variation of refractive index into the core of aspecial type of optical fiber using an intense ultraviolet (UV) sourcesuch as a UV laser. Two main processes are used: interference andmasking. The method that is preferable depends on the type of grating tobe manufactured. Normally a germanium-doped silica fiber is used in themanufacture of fiber Bragg gratings. The germanium-doped fiber isphotosensitive, which means that the refractive index of the corechanges with exposure to UV light.

The amount of the change depends on the intensity and duration of theexposure as well as the photosensitivity of the fiber. To write a highreflectivity fiber Bragg grating directly in the fiber the level ofdoping with germanium needs to be high. However, standard fibers can beused if the photosensitivity is enhanced by pre-soaking the fiber inhydrogen. More recently, fiber Bragg gratings have also been written inpolymer fibers.

FIG. 2 shows an exemplary chirped fiber Bragg grating fiber 200demonstrating effect on multiple wavelengths λ₁ 220, λ₂ 230, λ₃ 240, λ₄250, etc. in accordance with some embodiments of the disclosure providedherein. The refractive index profile 220 of the grating may be modifiedto add other features, such as a linear variation in the grating period,called a chirp. The reflected wavelength changes with the gratingperiod, broadening the reflected spectrum. A grating possessing a chirphas the property of adding dispersion—namely, different wavelengths,e.g., λ₁ through 4, reflected from the grating will be subject todifferent delays 210. This property has been used in the development ofphased-array antenna systems and polarization mode dispersioncompensation, as well.

Those in the art will appreciate that the result of a chirped FPGbroadens the spectrum of reflected light. Consequently, this produces ananti-dispersive relation in the time domain. However, it is noted thatgroup delays should also be accounted for. That is, time delay forλ₄>λ₃>λ₂>λ₁. The utilization of uniform and chirped FPGs will now bediscussed in detail.

FIG. 3 depicts an exemplary fiber Bragg grating sensing (FBGS) system300 comprising sensors 360, 375 and interrogator 310, in accordance withsome embodiments of the disclosure provided herein. Fiber Bragg gratingscan be used as direct sensing elements for strain and temperature. Theycan also be used as transduction elements, converting the output ofanother sensor, which generates a strain or temperature change from themeasured, for example fiber Bragg grating gas sensors use an absorbentcoating, which in the presence of a gas expands generating a strain,which is measurable by the grating. Technically, the absorbent materialis the sensing element, converting the amount of gas to a strain. TheBragg grating then transduces the strain to the change in wavelength.

Fiber Bragg gratings are finding uses in instrumentation applicationssuch as seismology, pressure sensors for extremely harsh environments,and as downhole sensors in oil and gas wells for measurement of theeffects of external pressure, temperature, seismic vibrations and inlineflow measurement. As such they offer a significant advantage overtraditional electronic gauges used for these applications in that theyare less sensitive to vibration or heat and consequently are far morereliable. In the 1990s, investigations were conducted for measuringstrain and temperature in composite materials for aircraft andhelicopter structures.

Heretofore, FBGS systems have been costly and delicate exhibiting alarge footprint. Specifically, interrogation sources typically require asource which can quickly sweep numerous frequencies and/or complexdetection devices, such as, optical spectrum analyzers (OSAs). Theinventor of the present disclosure has recognized a need for FPGSsystems which are robust, simple, and inexpensive while maintaining asmall footprint.

Turning to FIG. 3, the present disclosure contemplates a novel FBGSsystem 300 comprising a broadband light source 310, one of more fibersplitter/couplers and a plurality of optical detectors 360, 375. In oneor more embodiments, a controller (not shown) controls a broadband lightsource via current fed from conductive element 305. In the presentembodiment, the broadband light source 310 is a light emitting diode(LED) couple to a fiber 315. The light is launched from an LED directlyinto the fiber 315 placed extremely close to the LED surface. In someembodiments, the packaging described in the U.S. patent incorporated byreference allows us to directly place the fiber very close to the LEDand/or laser surface.

The efficiency of coupling from LED into a single mode fiber is quitesmall. For the case of proximity coupling we can estimate it as:

$\eta_{fib} = {\left( \frac{7\mspace{14mu}{um}}{150\mspace{14mu}{um}} \right)^{2}\left( {{Area}\mspace{14mu}{ratio}} \right)\frac{{\pi\left( {7.{Degree}} \right)}^{2}}{\pi}\left( {{Angular}\mspace{14mu}{Accptance}} \right)}$

And for the launched power we have:P _(L) =I _(led)η_(LED)η_(fib)

The above equation gives us roughly 10 nA/mA for a typical LED in thenear infrared (NIR). The inventor has recognized that despite the poorcoupling, the power transmitted, P_(L), is suitable as will described ingreater detail later in the disclosure.

Fiber 315 is removable attached to coupler 320. In one or moreembodiments, this feature allows for greater portability and flexibilityin the sale and application of the system. The coupler 320 is alsomechanically attached fiber 325 which then feeds light intocoupler/splitter 330.

After the coupler/splitter 330, only ½ of the light coupled into thefiber reaches the sensing FBG named FBG_(S) 340 via fiber 335. Since LEDis a broadband light source and FBG_(S) 340 reflects a small portion ofthe incident light spectrum (whose wavelength depends on thetemperature, strain etc.), we estimate the total intensity of thereflected light from the FBG_(S) 340 and into the detection system as:

$P_{G} = {\frac{1}{2}P_{L}}$

And,

$P_{R} = {\frac{1}{2}{P_{G}\left( \frac{\delta\;\lambda_{FBG_{s}}}{\Delta\;\lambda_{LED}} \right)}}$

where, P_(G) is the power incident on the FBG_(S) 340 after thecoupler/splitter 330 and P_(R) is the power of the reflected pulse fromFBG_(S) 340 after the splitter/coupler 330 but before incidence uponFBG_(D) 355 and splitter/coupler 350.

In the present embodiment, FBG_(S) 340 is a uniform fiber Bragg gratingand FBG_(D) 355 is a chirped fiber Bragg grating. One skilled in the artwill understand and appreciate that splitter/couples 330, 350 arefunctionally different depending on incident direction. That is, in onedirection light is coupled with another fiber, whereas the otherdirection splits the light into two egress fibers at the distal end.

Coupler/splitter 350 couples the light from fiber 345. Light propagatingthrough coupler/splitter 350 gets filtered by passing though FBG_(D)355. Those in the art can appreciated that the chirped FBG_(D) 355 actslike a band pass filter (or low/high pass filters on the sidebands).Thus, the received portion of the broadband LED incident uponphotodetector D1 360 will be used as a reference. A portion of P_(R) isreflected at FBG_(D) 350. This is the interrogated measurement whichpasses the light over fiber 365 to photodetector D2 375. The specificsof the in how these are used for calculation purposes will be discussedlater in the disclosure.

FIG. 4 illustrates exemplary spectra 400 from the source and resultantreflections at two different temperatures, in accordance with someembodiments of the disclosure provided herein. The bandwidth of theLED's spectrum 430 is of the order of 20-100 nm depending on the LEDtype while the bandwidth of the reflected light can be tuned fromfraction of a nm to 10's of nm.

In general, this ratio of bandwidths will be of the order of 1-5%. Thus,very conservatively, the reflected light into the detector system to theLED has effective efficiency of 25-100 pA/mA (included the detectorresponsivity of ˜0.25-1 A/W). This is illustrated in FIG. 4.

FIG. 4 shows a typical LED spectrum 430 in a solid line. At a firsttemperature, T₁, reflected light will be centered at λ−λ_(oLED)=0. At asecond temperature, T₂, reflected light will be centered at λ−λ_(oLED)=1nm. It is this transition which is the basis for strain and/ortemperature measurement and analysis.

FIG. 5 illustrates exemplary spectra from resultant reflections at twodifferent temperatures juxtaposed to broad band response of an exemplarychirped fiber Bragg grating fiber, in accordance with some embodimentsof the disclosure provided herein. The spectral graph 500 represents thespectral intensity 510 as a function of wavelength 520.

In the present embodiment, the solid line represents a typicalreflectance spectrum from broadband light 530. The detector systemitself will consist of an FBG with a reflectance profile that is quitebroad and covers the region of interest. The profile of which is shownin FIG. 5. These type of broad reflectance profiles can be constructedby using a chirped FBG, as in the present embodiment with FBG_(D).

Reflectance curve 540 represents bandwidth centered at a normalizedwavelength at lambda 0 at a first temperature. Since the reflectancespectrum from broadband light 530 and Reflectance curve 540 are known,these will be used as reference. Reflectance curve 550 representsbandwidth centered at a normalized delta wavelength at lambda˜0.2 at asecond temperature. Since the reflectance spectrum from broadband light530 is known, reflectance curve 550 will be used with the referencecalculation to estimate a lambda resulting from temperature, strain,etc., as previously described.

FIG. 6 demonstrates the decomposition of incident exemplary spectra 600on an exemplary chirped fiber Bragg grating juxtaposed to broad bandresponse thereof, the decomposition comprising transmission 660 andreflection 650, in accordance with some embodiments of the disclosureprovided herein. The spectral graph 600 represents the spectralintensity 610 as a function of wavelength 620.

As can be appreciated by on skilled in the art, this chirped FBG_(D)reflects wavelengths under the solid curve—albeit at attenuated levelsat the edges. This is called edge detection. The remaining power getstransmitted through FBG_(D) and detected at D₁ (see FIG. 4 forreference). The analog of the chirped FBG can be analyzed as a low passfilter in the frequency domain in the present disposition in thereflected direction.

FIG. 6 illustrates the filtering properties of FBG_(D) (solid line 630).Light incident on FBG_(D) (top dashed line 640) gets decomposed intolight reflected 650 and transmitted 660 (2^(nd) and 3^(rd) dashed lines,respectively). It can be seen that power is conserved and the reflectedpower is a function of the edge effects of the chirped FBG_(D). Theremaining power is transmitted and detected at D₁. In one or moreembodiments, the detectors are photon detectors (photodetector) in theinfrared or near infrared (NIR) spectra. In other embodiments, detectorare thermal detectors. However, any detector is not beyond the scope ofthe current invention.

As one can see, the ratio of intensity on the two detectors, D1 360, D2375 directly measures the wavelength of the reflected light from thesense FBG. A ratio such as:

$R = \frac{I_{D1} - I_{D2}}{I_{D1} + I_{D2}}$

Is independent of the LED's intensity variations as well as smallchanges in the wavelength spectrum. This ratio dies depend on theFBG_(D)'s transfer characteristics which must be made stable by design.

In another note, it is demonstrated that FBG_(D) itself can be monitoredand construct another “ratio of ratio” making it robust of the smallchanges in FBG_(D). The detecting FBG can be replaced by any filterincluding a grating or thin film optical filter.

Since we need to resolve wavelength shifts of the order of 10's pm (agood temperature sensing FBG is ˜10-15 pm/K), we can now estimate theSNR requirement. The change in the intensity of the two detectorsbecause of the small shift in the wavelength of the input light is:

${\delta\; I} = {\frac{d\; R}{d\lambda}\left( \frac{\delta\;\lambda_{s}}{\delta\; T} \right)\Delta\; T\; P_{R}}$

While temperature as an exemplary parameter, it can be replaced by anyother physical parameter that FBG is sensitive to. The term

$\frac{d\; R}{d\;\lambda}$represents the slope of the FBG_(D) and

$\frac{\delta\;\lambda_{s}}{\delta\; T}$represents the sensitivity of the sense FBGs. Again, let us make a quickdemonstrative estimate.

Since the total change in the wavelength of the reflected light may beof the order of a few nm (or 100's of K for temperature) we can safelyset

${\left. \frac{d\; R}{d\;\lambda} \right.\sim 0.1}\text{/}{nm}\mspace{11mu}\ldots$Thus, for a 1K resolution, we have:

$\delta\;{\left. I_{1K} \right.\sim\;\left( \frac{0.1}{nm} \right)}\left( \frac{0.01\mspace{14mu}{nm}}{K} \right)\left( \frac{1}{2} \right){\left. \left( \frac{25\mspace{14mu}{pA}}{mA} \right) \right.\sim 10}\frac{f\; A}{{mA}_{LED}\; K}$

It is noted that another factor of ½ from connectors etc. is includedand used to demonstrate the smallest (most conservative) estimatedreflected power from the sense FBG. Thus, we need a receiver systemwhose noise floor is of the order of 0.1-1 pA-rms as we can easily pump100 mA into the LED.

This is well within the capability of an exemplary analog front end(AFE) with a measurement bandwidth (BW) of the order of 10's of Hz. Thisis fast enough. In fact, given how conservative these estimates are,various optimizations from the LED to FBG's to coupling from LED to thefiber allows us another 2-3 orders of magnitude improvement over time.This can be traded off for higher BW of measurement or more sensitivemeasurement. The inventor of the present disclosure asserts 60-80 dB ofmeasurement SNR can be achieved with sufficiently low noise to make thesystem feasible.

In future, the detection system can be replaced by AWG or arraywaveguide grating to provide very compact spectrometer for multiplewavelengths monitored simultaneously. These are not beyond the scope ofthe current invention.

FIG. 7 depicts an exemplary fiber Bragg grating system 700 comprisingsensors 770, 780, interrogator 710, and optical circulators 730, 750, inaccordance an alternate embodiment of the disclosure provided herein. Inthe present embodiment, splitter/couplers are replaced with opticalcirculators 730, 750, which has the property to conserve power butgreatly increases the complexity and cost. However, any suitable opticaldevice, e.g., polarizing beam splitter, half-wave plate, half silveredmirror, etc., is not beyond the scope of the present invention.

In the present embodiment, FBG_(S) 740 is a uniform fiber Bragg gratingand FBG_(D) 760 is a chirped fiber Bragg grating. One skilled in the artwill understand and appreciate that optical circulators 730, 750 arefunctionally different depending on incident direction. That is, in onedirection light is coupled with another fiber, whereas the otherdirection splits the light into two egress fibers at the distal end.

Turning to FIG. 7, the present disclosure contemplates a novel FBGSsystem 700 comprising a broadband light source 720, one of more opticalcirculators 730, 750 and a plurality of optical detectors 740, 780. Inone or more embodiments, a controller (not shown) controls a broadbandlight source via current fed or other suitable circuit. In the presentembodiment, the broadband light source 720 is a light emitting diode(LED) couple to a fiber. The light is launched from an LED 710 directlyinto the fiber placed extremely close to the LED surface. In someembodiments, the packaging described in the U.S. patent incorporated byreference allows us to directly place the fiber very close to the LEDand/or laser surface.

After the optical circulator 730, light coupled into the fiber reachesthe sensing FBG named FBG_(S) 740. Since LED is a broadband light sourceand FBG_(S) 740 reflects a small portion of the incident light spectrum(whose wavelength depends on the temperature, strain etc.), we estimatethe total intensity of the reflected light from the FBG_(S) 740 and intothe detection system, similarly as previously described with or withoutof a ½ factor.

Light propagating through optical circulator 750 gets filtered bypassing though FBG_(D) 740. Those in the art can appreciated that thechirped FBG_(D) 760 acts like a band pass filter (or low/high passfilters on the sidebands). Thus, the received portion of the broadbandLED incident upon photodetector D1 770 will be used as a reference. Aportion of P_(R) is reflected at FBG_(D) 760. This is the interrogatedmeasurement which passes the light over fiber to photodetector D2 780.

FIG. 8 depicts an exemplary fiber Bragg grating system 800 comprisingsensors 850 and interrogator 810, in accordance with other embodimentsof the disclosure provided herein. In the simplest embodiment, theinventor has envisioned the implementation of a prefabricated system tobe marketed to previously laid FBG in various applications, e.g.,aero/astro, etc.

In the present embodiment, the interrogator 810 is an LED light sourceand current controller. Light is efficiently coupled into fiber 820 anddelivered to FBG_(S) 830. Reflected narrow band light is reflected intofiber 840, while power transmitted power through FBG_(S) 830 is absorbedand/or attenuation in a lossy medium.

Detector platform 850 receive the reflected light from 840. Detectorplatform 850 comprises photodetectors and an optical filter pursuant tothe present disclosure. That is, in the present embodiment, opticalcoatings are used to reconcile the reflected intensity with the overlapof the filter profile. This is known in the art as edge detection. Thepresent embodiment relies upon the profiles of the FBG_(S) 830 and theoptical coating to make such an empirical determination.

FIG. 9 depicts an exemplary fiber Bragg grating system 900 comprisingsensors 950, interrogators 910, an interrogated FBG_(S) 930 andreference FBG_(D) 930, in accordance with some embodiments of thedisclosure provided herein. Light is efficiently coupled into fiber 920and delivered to FBG_(S) 930. Reflected narrow band light is reflectedinto fiber 940, while power transmitted power through FBG_(S) 930 isabsorbed and/or attenuation in a lossy medium.

Light is equally efficiently coupled into fiber 960 and delivered toFBG_(D) 980. FBG_(D) 980 is used as a reference. In the presentembodiment, it divorces the need for the intimate correlation between aFBG_(D) and optical filtering (coating). Reflected narrow band light isreflected into fiber 970, while power transmitted power through FBG_(D)980 is absorbed and/or attenuation in a lossy medium.

Detector platform 950 receives the reflected light from 940 and 970 andperforms a much more accurate comparison using optical filters, overlapand edge detection. Detector platform 950 comprises photodetectors andan optical filter pursuant to the present disclosure. That is, in thepresent embodiment, optical coatings are used to reconcile the reflectedintensity with the overlap of the filter profiles and the referencesignal.

FIG. 10 depicts an exemplary fiber Bragg grating system 1000 comprisingsensors 1020, interrogators 1005, 1010, 1015 and a plurality of FBG_(S)1030, 1045, 1060 though a plurality of fibers 1025, 1040, 1055, inaccordance with some embodiments of the disclosure provided herein.Reflectance signals are received from fibers 1035, 1050, 1065.

In the present embodiment, the interrogator is a plurality of lightsources 1005, 1010, 1015 controlled by current controller. Light isefficiently coupled into fibers 1025, 1040, 1055 and delivered toFBG_(S) 1030, 1045, 1060. Reflected narrow band light is reflected intofibers 1035, 1050, 1065, while power transmitted power through FBG_(S)1030, 1045, 1060 is absorbed and/or attenuation in a lossy medium.

Detector platform 1020 receive the reflected light from fibers 1035,1050, 1065. Detector platform 1020 comprises photodetectors and anoptical filter pursuant to the present disclosure. That is, in thepresent embodiment, optical coatings are used to reconcile the reflectedintensity with the overlap of the filter profile. This is known in theart as edge detection.

In some embodiments, each of the plurality of light sources 1005, 1010,1015 is a different wavelength. This allows the return fibers 1035,1050, 1065 to be coupled together and reconciled by various filters atreceiver 1020. In other embodiments, all light sources 1005, 1010, 1015have the same wavelength but are pulsed at different time timestemporally, which also allow coupling into the same return fiber. Thiscan be reconciled in the time domain by an analog front-end.

In other embodiments, there is an assigned photodetector for each fiberlight sources 1005, 1010, 1015. In this configuration, the wavelengthcan be the same or manifold. Each can be accounted for by the opticalfilters and analog front-end (AFE) calculations. These will be describedin greater detail later in the disclosure.

FIG. 11 illustrates packaging for an exemplary fiber Bragg gratingsystem 1100 comprising sensors 1140, interrogators 1120 and AFE 1110, inaccordance with some embodiments of the disclosure provided herein. Thefiber Bragg grating system 1100 can be mechanically coupled viaalignment posts/reliefs 1130, 1150.

Package contains entire measurement System. The ADPD AFE manages timingto drive the LEDs as well as read the signals on the photodiodes,providing I2C data output. An external microcontroller can be used toprocess data, converting photocurrent values into temperature shifts.

FIG. 12 illustrates packaging for an exemplary fiber Bragg grating fiberinterface 1200, in accordance with some embodiments of the disclosureprovided herein. Fiber bundle 1210 can be mechanically couple to apreviously disclosed AFE package using alignment posts 1220.

Connection to FBG_(S) can be handled by a standard MTP style connectorbundle. The connector holds a linear array of optical fibers and has twopins for alignment. ADI can leverage pre-existing packaging technologyto easily allow connection to our package with 0.1 dB coupling loss.This also allows customers to provide their own fiber connectors, asopposed to ADI providing both the module and fiber optics.

FIG. 13 illustrates packaging detection for an exemplary fiber Bragggrating sensing system 1300 comprising sensors and optical filters, inaccordance with some embodiments of the disclosure provided herein. Inthe present embodiment, the coupled fibered is disposed at a distanceaway from the plurality of photodetectors to allow for the expansion ofthe fiber's beamwaist 1310.

Detectors 1330, 1340, 1350 are bereft of optical coating, in one or moreembodiments. Photodetector 1320 has been subjected to an optical coatingwhich serves the purpose of edge detection, as previously described.

In the preferred embodiment, the receiver is a differential measurementsystem. One photodiode is coated with a custom edge-pass filter, and thetemperature will be determined by the ratio of the filtered tounfiltered power.

As seen in the proceeding alternate embodiment, light is incident on afiltered photodiode at an angle and, with clever packaging optics, thelight reflected from the first photodiode would be captured by thesecond unfiltered photodetector.

To simplify design, however, the inventor of the present disclosure hasdetermined that the fibers should be mounted a set distance away fromthe photodiodes, allowing the beam waist to expand and illuminate boththe filtered and unfiltered photodiodes directly.

The largest problem with this sensing scheme is there is no way toseparate local vibration of the fiber from a spectral shift due totemperature change. To mitigate this, two or additional photodiodes1330, 1340, 150 will be used for each measurement. The two or moreadditional photodiodes will allow the device to measure the centerlocation of the beam, and cancel out any vibrations.

FIG. 14 shows an exemplary fiber Bragg grating sensing filter 1400, inaccordance an alternate embodiment of the disclosure provided herein. Inthe present alternate embodiment, an add/drop filter is used instead ofchirped FBG. In the present example, light 1410 comprising λ₁, λ₂, λ₃,etc. ingress the filter from the top at an oblique angle. λ₁ getsdropped out (filtered) 1430 through a dichroic mirror 1440 at the bottomof the wave guide 1420.

Consequently, λ₂ gets dropped out (filtered) 1450 through a dichroicmirror 1460 at the bottom of the wave guide 1420.

The remaining light propagate down the waveguide dropping subsequentwavelengths, λ₂, λ₃, etc. These filters are readily available andcommonly used in the telecommunications industry with very resolutionsand Os. However, any suitable optical device which can separate light isnot beyond the scope of the present invention.

In one or more embodiments, the LED may be modulated by (at or about)one or more carrier frequencies, e.g., in the order of GHz. Frequencymodulation (FM) is the encoding of information in a carrier wave byvarying the instantaneous frequency of the wave. This will allowreduction in the interference in the noisy environments. In someembodiments, the modulator multiplies the source signal with the carriersignal, thereby convolving the two in the frequency domain.

The signals electrically transduced signals from the optical detectorscan then demodulated by means and methods commonly known in the art.Specifically, the signal is deconvolved in frequency domain and filteredleaving on the source signal. Any of the standardmodulation/demodulation technique can be used, for example, LED may bemodulated at some freq. f and the signal at each of the detectorsdemodulated using standard demodulation techniques such as lock-inamplifier.

In some embodiments, fiber Bragg grating can be concatenated giving riseto multiport ports at different spatial locations. In one embodiment,these are uniform FBG_(S) centered at various wavelengths, i.e., λ₀, λ₁,λ₂, etc. This concatenation can be disposed at FBG_(S). In the contextof the embodiment using modulation, each wavelength is modulated by adifferent carrier thereby greatly mitigating interference amongwavelengths and carriers.

Correspondingly, a chirped concatenation can be used at FBG_(D). Inother embodiments, wavelength specific filters are used to split thesignal reflecting back from FBG_(S) into a tree configuration. Theresult is each color having its own photodetector. In the presentembodiment frequency modulation is used, however any type of modulation,such as, amplitude modulation where in envelope acts as the encoding, isnot beyond the scope of the present invention.

An aspect of the of the present disclosure is the source being an LEDbased light source with simple coupling. This has little to do with FBGor any particular optical transducer. Another aspect of the of thepresent disclosure is the read-out sensor that is relativelyinexpensive, based on the disclosed architecture, which can directlymeasure multiple channels and is easily extensible to multiple systems.Another aspect of the of the present disclosure is low-cost as the edgefilter can be either standard filters or FBG's themselves.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications, and improvements areintended to be within the spirit and scope of the technology describedin the application. For example, those of ordinary skill in the art willreadily envision a variety of other means and/or structures forperforming the function and/or obtaining the results and/or one or moreof the advantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the embodimentsdescribed herein.

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments described herein. It is, therefore, to be understood thatthe foregoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed. In addition, any combination of two or more features,systems, articles, materials, kits, and/or methods described herein, ifsuch features, systems, articles, materials, kits, and/or methods arenot mutually inconsistent, is included within the scope of the presentdisclosure.

The above-described embodiments may be implemented in any of numerousways. One or more aspects and embodiments of the present applicationinvolving the performance of processes or methods may utilize programinstructions executable by a device (e.g., a computer, a processor, orother device) to perform, or control performance of, the processes ormethods. In this respect, various inventive concepts may be embodied asa computer readable storage medium (or multiple computer readablestorage media) (e.g., a computer memory, one or more floppy discs,compact discs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other tangible computer storage medium) encoded with one ormore programs that, when executed on one or more computers or otherprocessors, perform methods that implement one or more of the variousembodiments described above.

The computer readable medium or media may be transportable, such thatthe program or programs stored thereon may be loaded onto one or moredifferent computers or other processors to implement various ones of theaspects described above. In some embodiments, computer readable mediamay be non-transitory media.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that may be employed to program a computer or otherprocessor to implement various aspects as described above. Additionally,it should be appreciated that according to one aspect, one or morecomputer programs that when executed perform methods of the presentapplication need not reside on a single computer or processor, but maybe distributed in a modular fashion among a number of differentcomputers or processors to implement various aspects of the presentapplication.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically, the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

When implemented in software, the software code may be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer, as non-limitingexamples. Additionally, a computer may be embedded in a device notgenerally regarded as a computer but with suitable processingcapabilities, including a Personal Digital Assistant (PDA), a smartphone or any other suitable portable or fixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that may be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that may be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audibleformats.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks or wired networks.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Elements other than those specificallyidentified by the “and/or” clause may optionally be present, whetherrelated or unrelated to those elements specifically identified. Thus, asa non-limiting example, a reference to “A and/or B”, when used inconjunction with open-ended language such as “comprising” may refer, inone embodiment, to A only (optionally including elements other than B);in another embodiment, to B only (optionally including elements otherthan A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) mayrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

As used herein, the term “between” is to be inclusive unless indicatedotherwise. For example, “between A and B” includes A and B unlessindicated otherwise.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

The present invention should therefore not be considered limited to theparticular embodiments described above. Various modifications,equivalent processes, as well as numerous structures to which thepresent invention may be applicable, will be readily apparent to thoseskilled in the art to which the present invention is directed uponreview of the present disclosure.

What is claimed is:
 1. A method for measuring the interrogation a fiberBragg grating comprising: illuminating a first and second photodetectorwith light from a light source; disposing a filter between the lightsource and first photodetector, the filter centered a first wavelength;filtering light incident on the filter; measuring a filtered firstintensity from the light incident on the first photodetector; measuringa second intensity from the light incident on the second photodetector;and calculating a change in the fiber Bragg grating using themeasurement of the filtered first intensity and second intensity.
 2. Themethod of claim 1, wherein the light source is an optical fiber.
 3. Themethod of claim 2, wherein the light has been reflected from the fiberBragg grating.
 4. The method of claim 1 further comprising calculating aratio of the filtered first intensity and second intensity.
 5. Themethod of claim 4, wherein an analog front-end performs the calculation.6. The method of claim 5, wherein the ratio is calculated by thefollowing: $R = \frac{I_{D1} - I_{D2}}{I_{D1} + I_{D2}}$ where I is thecurrent received from two detectors D₁ and D₂.
 7. The method of claim 1further comprising: illuminating a third photodetector with light fromthe light source; and measuring a third intensity from the lightincident on the third photodetector.
 8. The method of claim 7 furthercomprising: illuminating a fourth photodetector with light from thelight source; and measuring a fourth intensity from the light incidenton the fourth photodetector.
 9. The method of claim 8 further comprisingcalculating a center of a beam waist by comparing the intensities fromat least two of the photodetectors.
 10. The method of claim 9 furthercomprising calculating a ratio of intensities from at least two offirst, second, third and fourth intensities.
 11. The method of claim 9further comprising calculating a change in the fiber Bragg grating usinga plurality of the intensity measurements.
 12. A system for detectingshifts in wavelengths comprising: an optical element centered at asecond wavelength; a first photodetector; a second photodetector; awaveguide configured to pass light from the optical element to the firstand second photodetectors; and a filter centered at a first wavelengthdisposed in a light path between the waveguide and the firstphotodetector; wherein the first photodetector measures the filteredlight and the second photodetector measures unfiltered light.
 13. Thesystem of claim 12 further comprising an analog front-end configure toperform calculations on the measurements from the first and secondphotodetector.
 14. The system of claim 13 wherein the calculationsinclude estimating a ratio from the first and second photodetector. 15.The system of claim 14 wherein the ratio is calculated by the following:$R = \frac{I_{D1} - I_{D2}}{I_{D1} + I_{D2}}$ where I is the currentreceived from two detectors D₁ and D₂.
 16. The system of claim 12wherein the optical element is a fiber Bragg grating.
 17. The system ofclaim 16 wherein the calculations include calculating a change in thefiber Bragg grating using the measurement of the first and secondphotodetector and predetermined properties of the filter.
 18. The systemof claim 13 further comprising a third photodetector configured tomeasure unfiltered light, wherein the first, second, and thirdphotodetectors are disposed substantially in the same plane.
 19. Thesystem of claim 18 wherein the analog front-end is further configured tofind a center of intensity by comparing the intensities from at leasttwo of the photodetectors.
 20. An apparatus for measuring theinterrogation a fiber Bragg grating comprising: means for illuminating afirst and second photodetector with light from a light source; means fordisposing a filter between the light source and first photodetector, thefilter centered a first wavelength; means for filtering light incidenton the filter; means for measuring a filtered first intensity from thelight incident on the first photodetector; means for measuring a secondintensity from the light incident on the second photodetector; and meansfor calculating a change in the fiber Bragg grating using themeasurement of the filtered first intensity and second intensity.